Structure of the Thallus

Most lichens are more complex in structure. The algae are restricted to a particular layer in the thallus and besides the algal zone there is at least one other defined layer, the medulla, which contains no algae. Other layers, a cortex for example, may also be developed. These thalli with a stratified organization are called heteromerous (Fig. 93). 

In the algal layer the algae multiply by mitotic cell division and by aplano-spores. In Trebouxia, for example, the protoplast of an algal cell divides into several protoplasts, each of which subsequently secretes a cell wall. These aplanospores are freed by the rupture of the wall of the mother cell. The stages of this process can easily be observed in sections of the thallus. Sexual reproduction by zoospore formation has not been observed within the lichen 

In some fruticose lichens, such as Usnea which has a radial arrangement of the tissues, a central axial strand can be distinguished internal to the medulla (Fig. 46). The structure of the central axis is dense and consists of paraplectenchymatous or prosoplectenchymatous tissue giving considerable tensile or skeletal strength to the thallus. In other genera, i.e., Alectoria, Cladonia, and Ramalina, the central axis is absent. Its place can be taken by a central hollow or by gelatinous or spongy tissues. In Letharia a central cord is formed by fusion of several smaller strands (Fig. 29). 

In fruticose thalli which are held upright by the tube-shaped cortex, the cortical hyphae are either arranged netlike or periklin or they form a palisade tissue (Figs. 18, 27, and 28). 

All tissue types can build a lichen cortex. Occasionally, two different tissues form a cortex which then appears as a two-layered structure. For example, in Ramalina siliquosa the outer part of the cortex is formed by a few parallel to reticulately orientated hyphae, connected by anatomoses. The rest of the thick cortex is formed of hyphae with gelatinized walls with a fastigiate arrangement. The hyphae of the two tissues lie at right angles to one another (Fig. 31). 

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